1. Field of the Invention
The present invention relates to a heat treatment method and a heat treatment apparatus both of which are used for heating a thin plate-like precision electronic substrate such as a semiconductor wafer, a glass substrate for a liquid crystal display device, a glass substrate for a photomask and a substrate for an optical disk (hereinafter referred to simply as a “substrate”), which is implanted with impurities, by irradiating the substrate with flash light.
2. Description of the Background Art
Conventionally, a lamp annealer employing halogen lamps has been commonly used in the step of activating impurities in a semiconductor wafer after impurity implantation. Such a lamp annealer carries out the activation of impurities in a semiconductor wafer by heating (or annealing) the semiconductor wafer up to a temperature of, e.x., about 1000° C. to 1100° C. In such a heat treatment apparatus, the energy of light emitted from halogen lamps is used to raise the temperature of a substrate at a rate of about several hundred degrees per second.
Meanwhile, in recent years, with increasing degree of integration of semiconductor devices, it has been desired that the junction should be made shallower as the gate length is shortened. It has turned out, however, that even if the above lamp annealer, which raises the temperature of a semiconductor wafer at a rate of about several hundred degrees per second, is used to carry out the activation of impurities in a semiconductor wafer, there still occurs a phenomenon that impurities such as boron or phosphorous implanted in the semiconductor wafer are deeply diffused by heat. There is apprehension that the occurrence of such a phenomenon may cause the depth of the junction to exceed the required level, thereby hindering good device formation.
To solve the problem, U.S. Pat. No. 6,998,580 and U.S. Pat. No. 6,936,797 propose techniques for raising only the surface temperature of a semiconductor wafer implanted with impurities in an extremely short period of time (several milliseconds or less) by irradiating the surface of the semiconductor wafer with flashes of light from xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”). The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of the light emitted from the xenon flash lamp is shorter than that of the light emitted from the conventional halogen lamp, and it almost coincides with the fundamental absorption band of a silicon semiconductor wafer. Therefore, when a semiconductor wafer is irradiated with the flashes of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised quickly with only a small amount of light transmitted through the semiconductor wafer. It has also turned out that the flashes of light emitted within an extremely short period of time such as several milliseconds or less allow a selective temperature rise only near the surface of a semiconductor wafer. For this reason, such a temperature rise caused by using the xenon flash lamps in an extremely short time allows only the activation of impurities to be implemented without deep diffusion of the impurities.
Now, as a typical measure of the properties of semiconductor wafers implanted with impurities, used is a sheet resistance value Rs. Since the activation of impurities decreases a sheet resistance value on the surface of a semiconductor wafer, a lower sheet resistance value generally indicates that better activation of impurities is achieved. For this reason, a further decrease in the sheet resistance value is desired. In order to decrease the sheet resistance value, the surface temperature of a semiconductor wafer has only to be further increased.
In order to further increase the attained surface temperature of a semiconductor wafer to be still higher with the emission of flashes of light from flash lamps, however, it is necessary to emit flashes of light with greater irradiation energy in an extremely short period of time, which must result in an increase in the loads of both flash lamps and driving circuits therefor. Consequently, there also arises a problem of shortening the lifetimes of such flash lamps.
Further, since the intensity distribution of flash light in the surface of a semiconductor wafer is not completely uniform and fine patterns are formed on the surface of the semiconductor wafer, the inplane distribution of light absorptivity is not also uniform. Consequently, there is also variation in the inplane temperature distribution of the semiconductor wafer when the semiconductor wafer is irradiated with flashes of light.
A tendency is found that the intensity becomes higher in the peripheral portion of a semiconductor wafer than that in the central portion thereof also due to the effect of reflection on a chamber wall surface or the like. Consequently, there is also variation in the inplane temperature distribution of the semiconductor wafer when the semiconductor wafer is irradiated with flashes of light, with a tendency that the temperature is more apt to increase in the peripheral portion than in the central portion. Moreover, it is very difficult to cancel the variation in the inplane temperature distribution in a heat treatment with irradiation using flash light in an extremely short irradiation time.